In order to react to the strengthening of regulations of exhaust gas of a diesel engine in light duty vehicles, from the real driving emission (RDE) regulations, and the North American market, it is essential to decrease NoX. Particularly, a diesel vehicle needs to pass strict exhaust gas regulations, that is, the RDE regulation. According to a necessity in decreasing NoX, in order to purify NoX discharged from an engine, a lean NoX trap (LNT) or a selective catalytic reduction (SCR), that is, an exhaust gas purifying system, has been globally developed and applied. On the other hand, even though the light-duty diesel engine has excellent fuel efficiency, because of a high price of the diesel engine and an addition of the exhaust gas purifying system for reducing NoX, an exhaust gas purifying system, which has NoX purifying performance with high efficiency and is capable of decreasing material costs and weight, has been demanded. Accordingly, as a recently developed system in order to satisfy the demand, competitive companies have searched on S-DPFs, that is, systems, in which a diesel particulate filter (DPF) is coated with a selective catalytic reduction (SCR) catalyst, in which an SCR function is combined with a function of a DPF.
In this regard, Korean Patent Application Laid-Open No. 10-2015-0096329 (Method of Purifying Exhaust Gas of Diesel Engine) discloses a method of passing exhaust gas to be purified through an exhaust gas line including a diesel oxidation catalyst (DOC) converter for oxidizing gas-phase residual hydrocarbon (HC) and carbon monoxide (CO) to carbon dioxide (CO2) and partially oxidizing nitrogen monoxide (NO) contained in exhaust gas to nitrogen dioxide (NO2).
However, in the case of the SDPF positioned on an underfloor, during a start-off or in an idle condition during regeneration, a ring off crack may be generated in the filter by a temperature gradient difference between a temperature of a center portion inside the filter and a temperature of an outer portion inside the filter during the regeneration due to an excessive injection of urea.